In a spectrophotometric optical system of a microplate reader, an analog signal obtained by a photoelectric conversion should be ensured to match a range of an analog-to-digital converter (ADC) of a detection circuit. However, light of different wavelengths may carry substantively different amounts of energy. For example, an amount of energy carried by longer wavelength light might be 20 times as much as that carried by shorter wavelength light. Moreover, the light of different wavelengths share processing circuit modules, and these modules could not balance such an energy difference between lights of different wavelengths. Thus, some analog signals could not match with the range of the ADC, e.g. going beyond the limit of the range. In addition, a colorimetric element of a microplate reader generally has a plurality of colorimetric channels. In order to ensure a uniform output of the plurality of colorimetric channels, an even distributed light spot (i.e., energy of image) is required.
In order to solve the above-mentioned problems, the amounts of energy carried by light of various wavelengths in the light path shall be balanced so as to achieve substantively the same outputs. At this point, two kinds of methods might be used according to the prior art, which will be described hereinafter.
1) Adjust a gain value of an analog channel of the detection circuit rather than adjust an energy amount of the light, so as to realize an appropriate matching between the analog signal obtained via a photoelectric conversion and a range of the ADC of the detection circuit. However, as the gain value increases, a noise in the circuit also increases, thereby leading to a degraded detection accuracy.
2) Adjust energy amounts of the lights so that energy amounts of various wavelengths light are substantively the same, and thus it is no more necessary to greatly increase the gain value of the analog channel of the detection circuit. Accordingly, an excess noise due to a relatively larger gain value could be avoided and thus the detection accuracy may be improved.
For the above-mentioned latter method, the system could be incorporated with a cold mirror, an attenuator, an energy-matching type of optical filter, or a diaphragm with a center aperture. However, there are some shortcomings with any of the above-mentioned element, which will be described hereinafter.
I) For a technical solution of incorporating a cold mirror in the light path, the cold mirror will be positioned in 45 degrees with respect to an incidence light so as to turn the light travel path with 90 degrees. Thus, a suitable space room for accommodating the mirror should be designed. In addition, various parameters of the cold mirror need to be adjusted depending on a specific amount of energy carried by respective wavelength light. Accordingly, an increased cost and an enhanced failure risk will be caused.
II). For a technical solution of disposing an attenuator in front of an optical filter, the additional attenuator will notably increase a manufacture cost of the whole system.
III). For a technical solution of utilizing an energy-matching type of optical filter, it is problematic in that the process of applying a specific film to the optical filter is difficult, and moreover the optical filter with a relatively low light transmittance will has a relatively large failure risk.
IV). For a technical solution of incorporating a diaphragm with a center aperture, it is problematic in that the light spot is not even and a rim energy is low, and thus each fiber could not be assured to obtain an substantively equivalent energy.